In a number of industries it may be helpful to be able to coat a metal substrate, or to join together by welding, or to make repairs by welding processes of existing objects. The weld metal deposit may have certain desired qualities with a coating of a similar or dissimilar material. In some cases it may be desirable to coat a metal substrate with a ceramic surface coating. Sometimes the coating is added for wear resistance, or to replace or re-surface a worn member.
In the electro-spark deposition (ESD) process, a consumable electrode material is brought into contact with a metallic base surface to be treated to deposit a coating, which may be a ceramic coating, on the metallic substrate. Electro-Spark Deposition (ESD). One such application may pertain to welding electrodes for use in a production line. Welding electrodes are generally made of copper. The electrode may have a surface coating, such as a ceramic coating, that may be intended to increase electrode life. However, the substrate material need not be limited to welding electrodes or to copper.
ESD is a low-stress surfacing-hardening process that causes little distortion to the workpiece. ESD involves atomic-level metallurgical bonding of a discharge electrode coating material to the base metal by electro-spark discharge. ESD is a surface treatment process that improves the physical and mechanic properties of the surface of the base metal. As a form of micro-arc welding technology, ESD introduces a large current pulse during a capacitance discharge. A high temperature (5000˜25000 K) plasma arcing column melts or vaporizes a small part of the electrode rod coating material. The molten or vaporized electrode material is transferred to the surface of the base metal by this pulsed arcing micro welding. These traits permit ESD to be used in many surface treatment applications, including the surface coating of resistance spot welding electrodes. Successful ESD examples of resistance spot welding electrode coatings includes the surface in-situ deposition of TiC, TiB2, and TiB2-TiC. Welding electrodes with TiC, TiB2 or TiC—TiB2 coatings have been used in the spot welding of automobiles. ESD treated welding electrodes have made significant advances in industrial applications. coatings of vanadium-carbide, tungsten-carbide, titanium-diboride, zirconium-diboride, Titanium-carbide, Cr3C2, and so on, might be applied to various tool steels or aluminum, or other metals. However, there are some defects found in ESD coatings. For example, the grain in the heat affected zone (HAZ) near the coating layer may become coarse due to high thermal stresses arising during the ESD process (as shown in the Figures). Consequently, welding electrodes may not achieve the full potential life extension because of the flaws in the heat affect zone of the substrate matrix.
In the coating of welding caps using ESD technology, a TiC rod is connected to the positive terminal of a capacitor, and then brought toward the surface of a copper cap. The copper cap is connected to the negative terminal of the capacitor. Arcing occurs when they are brought close together. This raises the temperature in the arcing column and a molten droplet is produced at the tip of the TiC rod. The molten droplet will then be accelerated by the plasma jet and will strike the surface of the substrate forming a splash of TiC spot on the surface electrode material workpiece, and, if successful, the coating material will weld to the underlying substrate material. After many discharges, the surface will be covered in a layer of TiC coating. During the coating process, the molten droplets strike the substrate at a high velocity in the arcing column. As seen in FIGS. 18a, 18b and 18c herein, splashing may occur, resulting in cracks or delamination as observed in the coating layer. These defects may tend to reduce the product life of the caps to a great extent. Some researchers in university or industry have been trying to understand the cause of the coating defects and find ways to reduce them.
The surface area will be coated with a layer of the electrode material when swept by the electrode. The electrode cap may be mounted to a moving device. The condition of the contact may be dependent on the relative motion of the rod of depositing electrode coating material and the electrode cap to be coated.
Resistance spot welding is still the main technology used in the automotive assembly, especially in body frame construction. The life of welding electrodes has become an important issue in the resistance welding of galvanized steel and aluminum sheets. This in turn increases the consumption in automotive production and thus raises the production cost. To solve this problem, researchers conducted many studies. The most representative research result is the application of a protective coating layer (metallic or cermet) to the surface of welding electrodes through the electro-spark deposition process. This helps the welding electrodes to resist or delay alloy migration and plastic deformation during welding, thus improving the usage life of resistance welding electrodes.
Electro-spark deposition (ESD) is a micro-arc pulse welding technology which transfers electrode material to a metallic substrate with the use of high frequency and short duration current pulses. The main advantage of electro-spark deposition is the ability to produce metallurgical bonding between the coating material and the substrate base metal with low heat transfer. Due to thermal shock when the spark discharges, ESD coatings may tend to have flaws. FIG. 5 herein shows typical coating defeats (delaminations, porosity, cracks and uneven coating) of a welding electrode after the application of TiC coating using the ESD process.
Different types of processing technology have been tried by various domestic and foreign researchers to achieve grain refinement using the friction stir welding processing. Ultrasonic grain refinement processing, as a secondary process technology, has been widely studied and reported in areas such casting, welding, and surface material treatment. Kwanghyun Park was the first to study the ultrasonic assisted friction stir spot welding equipment and processes. Ultrasonic assisted friction stir welding process can produce welded joints with better performance than friction stir welding alone.
Friction stir processing technology is newly developed based on the friction stir welding process for the surface coating modification of composite material. Due to the unique thermal or mechanical characteristics, or both, friction stir processing has been used in the preparation and modification of surface coatings. Zhou Xiaoping et al (Chinese invention patent CN201010570898.8, the preparation and modification of Al2O3+TiB2+Al composite coating on aluminum surface by friction stir welding) has demonstrated that the density and micro-hardness of Al2O3+TiB2+Al composite coating produced by thermal spraying process can be improved by friction stir processing.
Similarly, Chinese invention patents (CN 201310050662.5) “A semi-solid ultra fine grain/nano-crystalline plate processing method based on ultrasound-assisted friction stir processing”, (CN 201310049003.X) “An ultrasound-assisted semi-solid friction stir processing method in a controlled low temperature environment”, and (CN 201310049927.X) “a realization of surface UFG/nano material based on ultrasonic assisted semi-solid friction stir processing method” have adopted ultrasound-assisted semi-solid friction stir processing technology, implemented with the use of a stirring pin for surface treatment. As a result, a friction stir process may not be suitable for the modification of surface coatings, such as those on resistance spot welding electrodes. In addition, the process is difficult, and power consumption may be high.
Resistance welding electrodes serve several purposes or functions: the conduction of welding current; application of closing pressure on the mating parts to be welded, and heat dissipation. The temperature of the welding electrode in contact with the workpiece is quite high, and the welding electrode itself generates heat when welding current flows due to its own internal resistance. The temperature on the top surface of the welding electrode may rise very quickly to a level that is only marginally lower than the weld nugget temperature.
In the view of the inventors herein, ultrasonic vibration may help to improve welding structure and performance in the welding and casting industries. Ultrasonic cavitation and acoustic streaming effects of ultrasonic vibration that may aid in refining grains in the heat affects zones of castings and weld pools. Chinese patent CN102019531A, which pertains to a portable ultrasonic assisted electro-spark deposition integrated repair and polish device and technology, suggests an ultrasonic approach. However, the ultrasonic excitation is added to modulation of the discharge electrode. This ultrasonic vibration applies only to the coating transfer of the deposition material. It appears to have little effect on the coating layer on the base metal of the workpiece.